Magnetic resonance imaging (MRI) systems may be used to diagnose and treat medical conditions by exploiting a powerful magnetic field and radio frequency (RF) techniques. When a subject of interest is exposed in a magnetic field, main field B0, individual nucleus spins in the subject tend to align with field B0, but still precess at the Larmor frequency. The subject may include any substance, tissue, organ, an object or body of interest, or the like, or any combination thereof. The overall motion of the nucleus spins in the subject may be simplified as net magnetization (M) which is the averaged sum of many individual nucleus spins. A second magnetic field, a radio frequency field (field B1), is applied to M, causing M to precess away from field B0. An induced current is generated due to the sweep of M past the RF coils in an MRI system. The induced current may be termed as a magnetic resonance (MR) signal. During imaging, the MR signals are given different phase encoding and different frequency encoding, according to the gradient magnetic field. The image may thus be reconstructed by two-dimensional or three-dimensional Fourier Transform.
The spin echo is an effect utilized to generate a series of echoes when an excitation RF pulse and a certain number of refocusing RF pulses are applied. A group of generated echoes are called an echo train. The number of echoes obtained in an echo train is called echo train length (ETL). A typical echo train is generated by the application of a 90° excitation RF pulse and a series of 180° refocusing RF pulses. However, due to the power deposition or specific absorption rate (SAR) caused by high frequency RF pulses, the temperature of the object or body (e.g., a tissue) may rise to a certain degree that may cause a tissue damage, and/or the reconstructed image may be blurry or have other artifacts. The application of variable flip angles may reduce power deposition, and may decrease the image acquisition time. The term “flip angle” may refer to the rotation of the net magnetization vector M by a radio frequency pulse relative to the main magnetic field B0. Thus, it would be desirable to develop a system and method for determining, improving, or optimizing a flip angle schedule applicable in MRI.